The present invention relates to a circuit arrangement of a control device for monitoring a voltage for voltage deviations above a specific voltage value, and for outputting a reset signal for interlocking of output elements that are controlled by the control device if a voltage deviation above the voltage value arises. The circuit arrangement includes an arrangement for generating a reference voltage, a first comparing element for comparing the reference voltage with a first comparison voltage value, which is derived from the voltage to be monitored multiplied by a first proportionality factor, and for outputting the reset signal, and a second comparing element.
A circuit arrangement is described in German Patent No. 40 23 700. This patent discloses a circuit arrangement for monitoring the frequency of a signal sequence of an electronic device, in particular of a microcomputer. The microcomputer controls output elements, e.g., the electrical components (sensors and actuators) of an internal combustion engine. The circuit arrangement for monitoring the frequency also includes a circuit arrangement for monitoring a voltage, e.g., the supply voltage of a microcomputer.
The circuit arrangement for monitoring a voltage that is known heretofore has a Zener diode for generating a reference voltage. The Zener diode is connected to a reference voltage, and also to ground via a resistor. The circuit arrangement also has a first comparing element, and a second comparing element that is designed as an operational amplifier. The first comparing element is used to monitor the supply voltage with regard to whether it exceeds an overvoltage threshold. The reference voltage is present at the positive input of the first comparing element. An overvoltage threshold which is picked off at a resistor of a voltage divider is present at the negative input of the first comparing element. The voltage divider is connected to the supply voltage and also to ground: By selecting the resistors of the voltage divider appropriately, it is possible to define a proportionality factor by which the supply voltage is multiplied in order to obtain the desired voltage value for the overvoltage threshold. The output for the first comparing element supplies a first reset signal for interlocking the output elements in the event of a problem, i.e., if the supply voltage exceeds the overvoltage threshold.
The reset signal is present at the positive input of the second comparing element. The reference voltage is present at the negative input of the second comparing element. A second reset signal for interlocking peripheral components is present at the output of the second comparing element. The second reset signal is only present if the reference voltage is greater than the voltage of the first reset signal.
Thus in the circuit arrangement that is known heretofore it is only possible to monitor a voltage with regard to whether it exceeds an overvoltage threshold. It is not possible to simultaneously monitor the voltage with regard to whether it is below an undervoltage threshold. Furthermore, in the circuit arrangement that is known heretofore, a problem arises when the circuit arrangement is powered up (Power-On-Fall), because as the supply voltage increases the two comparing elements remain in a non-defined state until their operating voltage is reached. This non-defined state of the comparing elements can cause the circuit arrangement for monitoring a voltage to function improperly.
An object of the present invention is to design and further improve the circuit arrangement of the type described above so that the circuit arrangement can monitor the voltage to be monitored with regard to whether it exceeds or, respectively, is below two independent and freely definable comparison voltage values.
To achieve this object, based on the circuit arrangement of the type described above, the present invention proposes that the second comparing element compare the reference voltage with a second comparison voltage value that is derived from the voltage to be monitored multiplied by a second proportionality factor, and proposes that the second comparing element output the reset signal.
The present invention recognizes the fact that, in order to monitor the voltage value to be monitored with regard to whether it exceeds an independent overvoltage threshold as well as with regard to whether it is below an independent undervoltage threshold, the two comparing elements cannot be coupled to one another.
For this reason, according to the present invention a separate comparison voltage value that is freely definable via the proportionality factor is present at each of the two comparing elements. The circuit arrangement according to the present invention monitors a voltage with regard to positive and negative voltage deviations that are outside a voltage window that is delimited by the two comparison voltage values. If the voltage that is to be monitored exceeds or is below the respective comparison voltage values, a reset signal for interlocking the output elements is present at the output of at least one of the comparing elements. In this way, if there is an overvoltage or undervoltage in the voltage to be monitored, is possible to keep the output elements from performing switching in a undefined manner due to the computer core operating in an undefined manner.
The voltage to be monitored is preferably the supply voltage of the computer core.
According to a useful further refinement of the present invention, it is proposed that the circuit arrangement have two voltage dividers which are connected to the voltage to be monitored and to ground, it being possible to pick off the comparison voltage values at a pick-off of each of the voltage dividers. It is possible to freely define the proportionality factor by choosing the resistors of a voltage divider appropriately. The comparison voltage value present at the comparing element can be adjusted by multiplying the voltage to be monitored by the proportionality factor.
Because in the circuit arrangement according to the present invention a separate voltage divider is assigned to each comparing element, the comparison voltage values present at the two comparing elements can be freely chosen, independently of one another. In this way it is possible to define any voltage window having an upper, overvoltage threshold and a lower, undervoltage threshold within which the voltage to be monitored is to lie in the normal case. As soon as the voltage moves outside this voltage window, the reset signal is activated and the output elements are interlocked.
Accordingly, according to a preferred embodiment of the present invention, it is proposed that the resistors of the voltage divider be chosen so that the first comparison voltage value at the pick-off of the first voltage divider constitute an overvoltage threshold, and the second comparison voltage value at the pick-off of the second voltage divider constitute an undervoltage threshold.
According to a particularly useful embodiment of the present invention, the comparing elements are comparators. A comparator is embodied, for example, as a high-sensitivity, non-negative-feedback difference amplifier. The output of a comparator that is connected in tri-state mode (open collector) is at a high potential (HI) when in idle state. Depending on how the inputs of the comparator are connected, the comparator switches to a low potential (LO) when a comparison voltage threshold is exceeded or when voltage is lower than a comparison voltage threshold.
It is advantageous that in the case of the first comparator the reference voltage is present at the positive input and the first comparison voltage value is present at the negative input, and in the case of the second comparator the reference voltage is present at the negative input and the second comparison voltage value is present at the positive input. The first comparator thus monitors whether the voltage to be monitored exceeds an overvoltage threshold. If so, the output of the first comparator switches to LO. The second comparator monitors whether the voltage to be monitored is below an undervoltage threshold and if necessary switches its output to LO.
The arrangement for generating a reference voltage are preferably embodied as a reference voltage diode.
According to a useful further refinement of the present invention, the collector of a transistor whose emitter is connected to the voltage to be monitored and whose base is connected to a first connector of a computer of the control device and is connected via a pull-down resistor to ground, is connected to the pick-off of the first voltage divider. Furthermore, it is proposed that the collector of a transistor whose emitter is connected to ground and whose base is connected to a second connector of a computer of the control device and is connected via a pull-down resistor to the voltage to be monitored, be connected to the pick-off of the second voltage divider.
The transistors that are connected in series with the voltage dividers ensure that when the supply voltage of the control device is powered up the comparators are in a defined state. During powering up of the control device, the supply voltage of the control device does not increase abruptly from one instant to the next but rather increases continuously over a fairly long period. As the comparators only work reliably at and above a certain operating voltage, there is a danger that the comparators will enter a non-defined state during powering up of the control device and before the operating voltage has been reached. By contrast, the transistors start working at an operating voltage that is significantly below the comparators"" operating voltage. Therefore during powering up of the control device, first the transistors start operating at their operating voltage and bring the comparators to a defined state. Furthermore, the transistors are connected so that they are in a defined state even if they have not reached their operating voltage.
According to another useful embodiment of the present invention, it is proposed that the computer of the control device send an initialization signal for enabling the output elements to the connectors to the circuit arrangement, provided the computer is in a reliable working state. As the comparators only start working reliably at a certain operating voltage, there is a danger that the comparators may take on a non-defined state during powering up of the control device and before the operating voltage is reached. To prevent this, in the case of the circuit arrangement according to the present invention the comparators do not work until initialization signals from the computer are present at the connectors to the circuit arrangement.